The present invention relates to a pressure sensor for measuring a pressure or a differential pressure related to a fluid flow. More particularly, the present invention relates to a pressure sensor using capacitive annular elements to measure pressure or the direction and flow rate of a fluid flow.
Pressure and fluid flow sensors can be used in many different applications. In industrial process control environments, for example, pressure sensors can be utilized to measure gauge pressure, absolute pressure and the like. Additionally, fluid flow sensors, for example, can be used to measure flow rates of process fluids and provide flow signals for flow indicators, controls, and flow volume metering. The term “fluid” as it is used herein refers to both liquids and gases, and their combination.
Differential pressure flow sensors measure the fluid flow rate in a pipe, vessel or conduit by measuring a pressure drop across a discontinuity within the pipe. One way to form the discontinuity is to place a flow restriction member or primary element within the pipe to produce the desired pressure drop. One such flow restriction member is an orifice plate that restricts the fluid flow and produces the measured pressure drop.
Typical flow rate measuring systems tap the pipe containing the fluid flow on either side of the flow restriction member, measure the pressure at each tap, and use an external pressure sensor to obtain the pressure drop. Impulse or gauge lines filled with fluid communicate the pressure at each tap to the external pressure sensor.
Such systems have relatively high installation costs due to the need to attach an external pressure sensor to the pipe. Moreover, it is necessary in such systems to provide additional leakage protection at the locations where the pipe is tapped, adding to the installation costs both in terms of installation time and material costs.
In some instances, the process fluid or gas being sensed by the pressure sensor may require a highly pure or ultra-pure environment. One technique to address such installation requirements is to separate the pressure sensor from the process fluid using an isolation diaphragm. Typically, an oil fill couples the pressure sensor to the isolation diaphragm such that pressure applied to the diaphragm is applied to the pressure sensor. However, this isolation technique can introduce errors in pressure measurements. Additionally, in highly pure process environments, if the diaphragm were to become damaged or the seal were to become damaged (either through corrosion or through collisions with particulate matter within the fluid flow), the entire process could become contaminated by the oil fill. In the case of silicon wafer fabrication, the flow meter must be dry, that is, with no fill fluid to transport the pressure signal.